1. Field of the Invention
The invention relates to an end supporting plate for a single crystalline ingot, particularly relates to improvement of an end supporting plate attached to a cylindrical single crystalline ingot during wire saw cutting of semiconductor wafers from the ingot.
Priority is claimed on Japanese Patent Application No. 2004-272339, filed Sep. 17, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
A wire saw is an apparatus for cutting an ingot. In wire saw cutting, cutting of a work piece (ingot) is performed by pressing the work piece to an array of running wires soaked with abrasive slurry (abrasive fluid of slurry state) suspending abrasive grains in lapping oil. Since a wire saw uses a very fine wire, it can slice thin semiconductor wafers from the ingot.
However, in conventional wire saw cutting, compared to semiconductor wafers sliced from central barrel portion of an ingot, wafers sliced from end portions of the ingot could not have escaped from remarkable occurrence of nanotopography (waviness in finite area) and/or warpage.
A capable cause of nanotopography and warpage is a thermal on or contraction of groove rollers by temperature variation during cutting process. Temperature by heating of a bearing for rotating and supporting the groove roller, and processing heat of the wire and abrasive slurry during grinding. By this temperature elevation, locus of wires drift towards both ends of the groove roller.
Based on the extensive investigation on the occurrence of nanotopography and warpage of semiconductor wafers sliced from single crystalline silicon ingots, the inventor found the method described in Japanese Unexamined Patent Application, First Publication No. 2004-1409. In that method, dummy plates are attached to both ends of a cylindrical single crystalline ingot. The dummy plates are glass plates having cylindrical shapes of approximately same diameter with the ingot. Alternatively applicable form of glass plate has a predetermined thickness and approximately similar size with the ingot diameter in the cutting direction, and in its vertical direction.
The single crystalline ingot attached with the dummy plates are sliced to semiconductor wafers by a wire saw. By attaching the dummy plates to the ingot, the end portions of the ingot being sensitive to adverse effect of nanotopography and warpage is replaced by dummy plates. Therefore, the occurrence of nanotopography and warpage can be effectively reduced in the silicon wafers.
A wire saw is also used for slicing of semiconductor wafers with a diameter of 300 mm. In order to reduce a calf loss (cutting loss) of the semiconductor wafer, it has been investigated the reduction of wire thickness and use of fine size of abrasive grains.
However, the glass end supporting plates are harder than a single crystalline silicon ingot. When an ingot attached with the above described end supporting plates is sliced by a wire saw, along with decreasing grain size of free abrasive grain, grinding effect of the abrasive grain for a work piece pressed to the wire is also reduced. The reduction of grinding effect causes a difference in cutting speed of the end supporting plate and the ingot, increases load on the wire, and results in breaking of the wire.
The present invention was carried out based on the above consideration and its objective is to provide an end supporting plate which can reduce nanotopography and warpage of semiconductor wafers after slicing, and also to provide an end supporting plate which can protect the wire from breaking due to the use of fine (high number of) grain size of abrasive grains in the wire saw cutting.